Water meters may be divided into two large categories: volumetric meters and speed meters. Volumetric meters, e.g. meters having a rotating piston, enable very good sensitivity to be obtained at low flow rates, but their manufacturing cost is relatively high because they require highly accurate machining.
Speed meters may be sub-divided into Woltman type meters in which liquid flow rotates a propeller whose axis of rotation is parallel to the direction of liquid flow, and jet meters in which the liquid to be measured enters a measurement chamber perpendicularly to the axis of rotation of a turbine.
Woltman type meters are well adapted to metering liquids flowing in large bore pipework (typically not less than 80 mm in diameter) whereas jet type meters are used in conjunction with medium and small diameter ducts.
Jet type meters may further be sub-divided into single jet meters and multiple jet meters. In multiple jet meters the water inlet pipework opens out into an annular feed chamber. The measuring chamber in which the turbine is mounted is separated from the feed chamber by a cylindrical wall having orifices therethrough. The measurement chamber is thus fed by a plurality of jets created by these orifices, which jets cause the turbine to rotate. Multiple jet meters have good sensitivity, but their manufacturing cost is relatively high because of their relatively complex structure.
In single jet meters, the upstream pipework opens directly into the measuring chamber via an injector. The structure of such meters is thus much simpler and their cost is reduced. However, meters of this type generally suffer from mediocre sensitivity at low flow rates, and they can only be used as subsidiary meters because of their mediocre measuring qualities.
French published patent specification No. 2 336 666 seeks to remedy this drawback, i.e. to provide a single jet meter which has adequate measuring qualities to be used as a main or principal meter, by using a special method of pivoting the turbine to limit the friction torque at low flow rates, i.e. at low speeds of turbine rotation. According to that patent specification, the bottom end of the turbine's vertical shaft is terminated by a fine point, while its top end is terminated by a flat abutment surface.
At low flow rates, the shaft is supported by its fine point engaging a jewel which ensures sensitivity because of the low friction torque. As the flow rate increases, the bottom point lifts progressively out from the jewel until the turbine is completely lifted and has its top abutment surface pressing against a thrust bearing of suitable dimensions for the thrust involved. The turbine is lifted because of a vertical component in the resultant of the forces applied to the turbine. This component is increased by the presence of radial ribs projecting from the bottom of the measuring chamber.
Although this solution serves to reduce the friction torque at low flow rates, and thus to improve meter sensitivity in this operating range, it nevertheless suffers from the drawback of requiring a strong top bearing capable of absorbing the axial thrust from the turbine when operating at high speeds of rotation.
An object of the invention is to provide a single jet meter in which the turbine bearings are further improved so as to enable good sensitivity, particularly at low flow rates, while retaining low manufacturing costs compatible with mass production.